The genetic information encoded in DNA is packaged and interpreted by the cellular machinery within the context of chromatin. As the genome sequence remains largely unchanged throughout development, chromatin modifications represent a critical interface between the genome and regulatory inputs. Histone proteins are major constituents of chromatin; four core histone types make up the nucleosome particle, the basic unit of chromatin. The fifth type, linker histone H1, binds the nucleosome and the linker DNA between. Whereas the progress in understanding the function of core histones is significant in recent years, much of the linker histone H1 biology remains unknown. Mutations in linker histone H1 have been recently reported in approximately 30% of follicular lymphomas and diffuse large B cell lymphomas, and we have identified H1 mutations in 85% of Hodgkin's lymphomas of the nodular sclerosis subtype. Based on preliminary data, we hypothesize that H1 stem restricted dependency clones. B occurring causing cooperates overarching immune this control to tumor isoform loss of function induces lymphomagenesis by ectopically inducing an embryonic cell (ESC) gene expression signature that imparts unlimited self-renewal in GC B-cells that are otherwise in their ability to proliferate. We propose that ESC gene expression frees B-cells from their on T-cell help to survive and divide, resulting in expansion of aberrant pre-neoplastic B-cells Mechanistically, we propose that H1 is involved in PRC2 recruitment and chromatin compaction during cell differentiation, thereby repressing ESC genes. Thus, loss of function of H1 through somatic mutations in germinal center B cells results in reduction and redistribution PRC2 and H3K27 methylation chromatin decompaction and expression of ESC genes. Finally, we predict that H1 loss of function with lymphoma oncogenes such as BCL2 to induce malignant transformation. The goa of the proposed research is to reveal the contributions of linker histones to the humoral response and lymphomagenesis from both biochemical and biological perspectives. We will achieve through the following specific aims: 1) Determine the role and mechanism through which H1 is required to the GC reaction; 2) Determine the mechanism through which H1 mutations reprogram the epigenome drive lymphomagenesis; and 3) Determine whether and how H1 soforms function as bona fide lymphoma suppressors. The study of H1 is quite challenging, but we are uniquely poised to unravel GC canonical l i the molecular details and functional consequences of H1mutations in lymphoid carcinogenesis by bringing together leaders in the field of transcriptional regulation, chromatin, epigenetic regulation and lymphomagenesis.